This project is directed toward defining and preventing changes in platelets that occur during extracorporeal circulation. Work just completed shows an important relationship between priming volume and surface area ratio is 1 ml/1-2cm2 minimal changes in human platelet numbers and function occur during 6 hours of extracorporeal recirculation. When the ratio is 1 ml/19cm2 platelet numbers decrease and remaining platelets do not respond to aggregating agents and many have released platelet specific factor 4 antigen. If prostaglandin E1 is added to blood, most to the changes that occur during extracorporeal recirculation are prevented.